1. Field of the Invention
This invention relates to a laser system such as a broad area semiconductor laser, a phase-locked semiconductor laser or an optical waveguide type laser which uses a light amplifier medium having a transverse mode control mechanism comprising an optical waveguide layer which controls the transverse mode in one direction, and more particularly to a laser system in which oscillation in only a fundamental transverse mode is efficiently obtained.
2. Description of the Related Art
Lasers have been employed as a light source for a scanner in various fields such as a scanner for recording information on a recording medium or reading information recorded on a recording medium, and there is a demand for a higher output laser.
As a method of increasing the output power of a laser, there has been known a method in which the active area or the oscillation mode volume of the light amplifier medium is increased. For example, the area in the transverse direction of the light amplifier medium may be increased or gain of the light amplifier medium may be increased.
This approach will generally encounter the difficulty that a plurality of high order transverse modes are generated and it is impossible to obtain a single transverse mode. Accordingly the emitted laser beam cannot be condensed to a diffraction limit and the power density of the condensed laser beam cannot be increased with increase in the oscillation output power. This problem is remarkable especially in semiconductor lasers which are high in gain. As a high output semiconductor laser, there has been known a broad area semiconductor laser in which the width of the active area is enlarged. However since, in the broad area semiconductor laser, the resonator is not provided with modal discrimination function to cause oscillation in a single transverse mode, a plurality of high order transverse modes are simultaneously generated and accordingly the emitted laser beam cannot be condensed to a diffraction limit and a beam spot of high power density cannot be obtained.
In order to enhance modal discrimination to overcome these problems, there have been proposed various resonator structures as follows.
1) Space Filter System
There has been known a system in which only a lowest order fundamental transverse mode is effectively oscillated by increasing loss in high order modes by providing a space filter, comprising a lens and a simple opening, in the resonator. See, for instance, "High-Radiance Room-Temperature GaAs Laser With Controlled Radiation in a Single Transverse Mode" (E. M. Phillip-Rutz, IEEE J. Quantum Electron QE-8, 632(1972)), "High-power, diffraction-limited, narrow-band, external-cavity diode laser" (W. F. Sharfin, J. Seppala, A. Mooradian, B. A. Soltz, R. G. Waters, B. J. Vollmer, and K. J. Bystrom Appl. Phys. Lett. 54, 1731(1989)), PCT Japanese Publication No. 4(1992)-504930, and "Flattening of the spatial laser profile with low losses and minimal beam divergence" (V. Kermene, A. Saviot, M. Vamppouille, B. Colombeau, C. Froehly, and T. Dohnalik, Opt. Lett. 17,859(1991)).
However in such a laser, since a simple opening is employed, high order mode spatial frequency components cannot be expelled from the resonator though loss in high order modes can be relatively increased. Accordingly, when loss is increased and gain is enhanced, natural oscillation mode of the resonator can be obtained, and as a result, oscillation in high order modes cannot be suppressed in light amplifier media having sufficiently high gain such as semiconductor lasers and a plurality of high order transverse modes are generated as well as a fundamental transverse mode.
2) Mode Selective Mirror System
There has been known a system in which only a desired spatial mode is caused to oscillate by use of a resonator mirror having mode selectivity. As a laser employing this system, there has been known, for instance, a diffraction type mode selective mirror system laser. See, for instance, "High modal discrimination in a Nd:YaG laser resonator with internal phase gratings" (J. R. Leger, D. Chen, and K. Dai, Opt/ Lett. 19, 1976(1994)) and "Large-area, single-transverse-mode semiconductor laser with diffraction-limited super-Gaussian output", (G. Mowry, and J. R. Leger, Appl. Phys. Lett. 66,1614(1995)). However, this system involves the problems similar to those in the space filter system.
3) Inverse External Cavity System
There has been known a system in which the phase of a laser beam radiated from a light amplifier medium is inverted and the inverted phase laser beam is reintroduced into the light amplifier medium. See, for instance, "Laser mode control using an inverse external cavity", (C. C. Shih, SPIE Proceedings 2889,410, (1996)). Also in this system, it is not possible to cause only the fundamental transverse mode to effectively oscillate since oscillation in even high order transverse modes cannot be suppressed though oscillation in odd high order transverse modes can be suppressed.
As can be understood from the description above, in accordance with the prior arts, though the laser system using a laser medium having a wide active area or a large oscillation mode volume can be increased in its output power, it cannot radiate a high quality laser beam oscillating only in a fundamental transverse mode. Accordingly, when the output power is increased, the emitted laser beam cannot be condensed to a diffraction limit, which makes it difficult to obtain a high power density.